Photodetectors are an integral part of optical circuits and components (for example emitters, modulators, repeaters, waveguides or fibers, reflectors, resonators, detectors, IR Focal plane arrays, UV microchannel arrays and THZ diode detectors etc.) and are used for the sensing of electromagnetic radiation. There are several approaches to these devices. Photoconducting materials, typically semiconductors, have electrical properties that vary when exposed to electromagnetic radiation (i.e. light). One type of photoconductivity arises from the generation of mobile carriers (electrons or holes) during absorption of photons. For semiconducting materials, the absorption of a specific wavelength of light, hence photon energy, is directly proportional to the band gap of the material (Eg=hn=hc/l, where Eg is the materials band gap, h is Plank's constant (4.136×10−15 eVs), c is the speed of light in a vacuum (2.998×1010 cm/s) and l is the wavelength of the radiation). If the band gap energy is measured in eV (electron Volts) and the wavelength in micrometers, the above equation reduces to Eg=1.24/l. A photodiode (i.e. p-n diode, p-i-n photodiode, avalanche photodiode, etc.) is the most commonly employed type of photoconductor.
Light detection is ideally suited for direct band gap semiconductors such as Ge and GaAs. However, indirect band gap semiconductors (where an additional phonon energy is needed to excite an electron from the valence band to the conduction band), such as Silicon, are also used as photodetectors. A widely known type of photodetectors is the solar cell, which uses a simple p-n diode or Schottky barrier to detect impinging photons. Besides silicon, most photodetectors disadvantageously do not integrate with existing microelectronics technology, usually detect only a specific wavelength (i.e. 1.1 mm for Si, 0.87 mm for GaAs, 0.414 mm for a-SiC and 1.89 mm for Ge), and require multiple detectors to detect a broad band of wavelengths (hence photon energy).
Besides photodiodes, there are other types of photodetectors that do not rely on the generation of current through the excitation of electrons (or holes). One type of detector is the bolometer. Bolometers operate by absorbing radiation, which in turn raises the temperature of the material and hence alters the resistance of the material. For useful background information on bolometers and semiconductor devices, refer to Kwok K. N G, “Complete Guide to Semiconductor Devices,” IEEE Press, John Wiley & Sons, 2002, pages 532-533. Bolometers can be constructed from metallic, metallic-oxides or semiconducting materials such as vanadium oxide and amorphous silicon. Since bolometers detect a broad range of radiation above a few microns, bolometers are typically thermally stabilized to reduce the possibility of detection of blackbody radiation that is emitted from the detector material, which leads to a high background noise. Unlike other detector technologies, IR microbolometer detectors and arrays advantageously do not require cooling to cryogenic temperatures unlike the other detector technologies discussed.
Typical band-gaps for carbon nanotubes (CNTs) range from approximately 0.6-1.2 eV, depending on the diameter of the CNT, where the band gap is proportional to the inverse diameter of the nanotube. These energies correlate to the ability of the nanotubes to detect radiation in the near IR range. Since nanotubes also generate heat and phonons by several processes (e.g., injection of electrons, impinging with radiation, etc.), a CNT fabric is also ideally suited as an IR detector. For graphene, which has a zero electron volt band gap, high mobilities (approximately 100,000 cm2/Vs) and carrier saturation velocities on the order of approximately 5×10E7 cm/s, the nanoribbons can serve as either photodetectors or a microbolometer through modulation of the temperature coefficient of resistance of the graphene layer(s).
An existing prior art microbolometer utilizes vanadium oxide as the element which changes impedance for incoming IR radiation. Typically 2% per degree C. is the highest thermal coefficient of resistance achievable. This performance is limited by 1/f noise and the basic physical properties of the vanadium oxide film. The VOx based micro bolometer is fabricated on top of the CMOS readout circuit, which provides a cost benefit.
Accordingly, it is desirable to provide carbon nanotube based UI, IR and THZ radiation and light detecting systems to enhance overall sensitivity of the system.